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Lecture 2: Biomaterials Surfaces: Physics
The surface of a material strongly dictates its performance in vivo.
Surface Properties Influencing Cell Adhesion
Wettability
Crystallinity
Roughness
Composition
Electrical Charge
Mobility
What’s so special about a surface?
Surface vs. Bulk
1015 atoms
1. Inherently Small # of Atoms
Requires special characterization tools
2. Enhanced Mobility


fewer bonds
gradient in density
Facilitates rate-limited processes
(phase transformations, crystallization, corrosion…)
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Example: Devitrification of calcium phosphate glass CaO-P2O5-SiO2 (44:40:15)
（devitrification）]
Crystallization initiates at surface
from J.-S. Lee et al., J. Thermal Anal. Cal. 56 (1999) 137.
3. Higher Energy State
Atoms/molecules with unsatisfied (“dangling”) or strained bonds
High reactivity and susceptibility to adsorbates
Quantifying Surface Energy
loss of bonds at a surface
attraction towards bulk
= a real contraction force
Surface tension, γ, is the work required to create unit surface area at constant T,P and
composition.
Consider a simple soap film experiment:
where G = Gibbs free energy, A = area
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Surface Tensions of Example Materials
Trends:
highγmaterials: (>200 dyn/cm) – metals, carbides, oxides
low γ materials: polymers, organics
Why? Consider the nature of bonds…
Surface tension is a measure of degree of cohesion.
Work of cohesion: WC = 2γ
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Surface Phenomena
A simple rule: Surface phenomena are driven primarily by an
associated reduction in surface free energy.
Important examples in biomaterials:
adsorption of a species from environment

surface segregation of a species from bulk

surface reconstructions

surface reactions
1. Adsorption phenomena
Tenet 1: Higher energy surfaces are quickly coated/contaminated by lower energy
species.

Examples: * Water on glasses, metals or oxides
* Hydrocarbons on inorganic surfaces
Measured of metals & oxides ~ 37 dyn/cm
* Surfactants at air/water interface
Classes of adsorption:

chemisorption – strong modifications to electronic structure/electron density of
adsorbate molecule (> 0.5 eV/surface site)
Example: H2O on silica
eV
Eads = 1.7 eV O
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 physisorption – adsorbate weakly adherent via secondary
(i.e., van der Waals’) interactions (< 0.25 eV/surface site)
Example: PMMA on silica
Eads = 0.1 eV/mer
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Tenet 2: The “high energy surface” of tenet 1 is relative to its surrounding
medium.
In H2O based environments, a hydrophilic material has a lower
interfacial energy than a hydrophobic one.
Example: Adsorption/denaturing of proteins on hydrophobic surfaces in
water-based environments
+ _ Charged
& H-bonding groups orient towards H2O; hydrophobic groups orient
towards polyethylene
+
Such adsorption phenomena are examples of “thermodynamic adhesion”
Adhesion – state in which 2 dissimilar bodies are held together in intimate
contact such that a force can be transferred across the interface.
Thermodynamic adhesion is driven by interfacial forces associated with
reversible processes.
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Work of Adhesion (W12): the work required to separate a unit area of interface between
2 phases.
Suggests 2 strategies for protein resistance: W12= 0
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The hydrophilicity of a surface can be gauged by measuring the contact angle of a
droplet of water on the surface. The balance of interfacial forces is described by
Young’s Equation:
For multi-component surfaces:
2. Surface Segregation
An interfacial adsorption phenomenon involving a bulk component of a
multi-component material.
Example 1: Surface segregation of a dilute solute (B) in a binary AB alloy
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Empirically for many alloys, Hondros found:
For metals with multiple solutes, highest activity solute
generally dominates the surface.�
Surface fraction of B (XB,S) can alternately be described using the
Langmuir-McLean relation:
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,
The predicted behavior is more transparent by expanding the exponential
The Langmuir-McLean relation indicates:
Surface enrichment occurs when △GS is negative�
Surface coverage increases with bulk solute content�

Surface enrichment decreases with increasing T�
GS can be estimated from the Miedema eqn:


prefactor is fraction of atom contacting vacuum
molar volume 莫耳體積
The Miedema model is ~90% accurate in predicting segregation in AB alloys.
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Surface segregation also occurs commonly in organic materials…
In polymers, ENTROPY can play a significant role in surface segregation.
Chain ends surface segregate to decrease entropic penalty
Polymer “random coil”conformations are restricted by the presence of a surface
Less chain configurations!
Short chains surface segregate when mixed with long chains
短鏈(高分子)與長鏈(高分子)混合時會在表面偏析
Surface segregation importance to biomaterials applications:

Toxicity

Corrosion resistance

Modified protein/cell adhesivity
Surface modification with a comb polymer additive
But…also a strategy for surface modification!
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3. Surface Reconstruction
Atomic or molecular rearrangement at surface to reduce surface/
interfacial tension.
Example 1: Faceting in MgO
(100) is preferred plane of cleavage (charge neutral!)
Miscut surfaces will facet
1) no net dipole moment
2) minimial loss of nearest neighbor ligand coordination
Low Energy Oxide Surfaces
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Example 2. Reorientation of polymer chains in water vs. air
Chain reorientation can be observed with dynamic contact angle studies:
Advancing: Droplet volume increased (by syringe)